There are applications for which it is desirable to have an optical bandpass filter with two distinct passbands. One approach would be to fabricate a multi-layer coating that has two separate passbands. But as a practical matter, it can be difficult to actually manufacture such a coating with two separate passbands that each have the desired frequency range. Further, the spacing between the passbands becomes fixed at the time of manufacture. In addition, although the filter can be tilted from normal incidence in order to achieve a degree of tuning, this causes both passbands to move simultaneously toward shorter wavelengths. It is not possible to significantly vary the center wavelength of one passband without affecting the other passband, nor is it possible to vary the bandwidth of either passband. Thus, although existing dual-passband optical filters have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects.